Effect of >CO···H–Ar interaction on endo/exo selectivity in the Diels–Alder reaction of phenyl-substituted cyclopentadienones

The cycloaddition of 2,5-bis(methoxycarbonyl)-3,4-diphenylcyclopentadienone with S-allyl S-methyl dithiocarbonate gave the endo cycloadduct whereas phencyclone gave a mixture of the endo and exo cycloadducts. The X-ray analysis of the cycloadduct of phencyclone and S-allyl S-methyl dithiocarbonate indicated the presence of a short contact of Ar–H···OC< type. A possible role of the interaction in determining the endo/exo selectivity was discussed based on the X-ray crystallographic data and the transition structures from the MO calculation data.